Process for producing electrode paste



Nov. 20, 1962 o. H. SANDBERG PROCESS FOR PRODUCING ELECTRODE PASTE Filed Nov. 23

/ GRAPHITE CONTENT INVENTOR. OVE HELGESS'O'N SAND BY BERG az Wm ,4 5&1

ATTORNEY-3.

United States Patent 3,065,094 PRGCElS FGR PRODUCING ELECTRODE PASTE Ova Helgessiin Sandherg, Oddernes, pr. Kristiansand, Norway, assignor to Elektrokemisk A/S, Oslo, Norway, a

corporation of Norway Filed Nov. 23, 1959, Ser. No. 854,662 Claims priority, application Norway Nov. 26, 1958 3 Claims. (Cl. 106--284) This invention relates to the production of electrode paste for carbon electrodes in electric smelting furnaces. Such paste may for example be used in self-baking electrodes of the so-called Soederberg type which are baked during operation of the furnace itself.

The electrode pastes heretofore have been produced by crushing calcined anthracite (with the addition of some metallurgical coke in some instances) and sifting it to definite grain fractions which are then mixed in definite proportion by weight with a carbonaceous binding agent such as tar or pitch. As stated above, a Soederberg electrode is baked gradually in a furnace in which it is used, the baking taking place as the electrode is lowered gradually through the holder to compensate for electrode consumption. In such case, during baking, volatile components are expelled from binding agent which is thereby coked and the electrode mass becomes solid and hard and acquires at the same time the required electrical conductivity.

Particularly in the case of the continuous Soederberg type electrodes, the electrode during its hardening is being subjected to mechanical strains such for example as bumps from mechanical stokers and bending during tilting of the furnace and the like. It is therefore important that the electrode have a high resisivity against bending and a low modulus of elasticity. It is also essential that after baking the electrode must have good electric and thermic conductivity.

Heretofore it has been found that trying to improve one of these four characteristics has involved a sacrifice of the other three and the production of the electrodes has been a compromise to get the best average characteristics and it has always been considered advisable to carry on the calcination of the carbonaceous material in such a way that substantially all the volatile materials were driven out but with a minimum of graphitization prior to the final baking. This was considered desirable in order to get proper strength.

The present invention is based upon my discovery that actually greatly improved electrodes can be made if in place of simply calcining anthracite coal one uses petrol or equivalent coke and carries on the calcination far enough so that there is a measurable and appreciable degree of graphitization before the final bonding.

The degree of graphitization of materials of this type can be measured by the conductivity. To this end the material to be tested is ground to a powder which will pass through a 20 mesh screen and be retained on a 70 mesh screen. Such powder is introduced into a cylinder of 30 mm. internal diameter which is filled up to a height of substantially 25 mm. The material in the cylinder is then put under an actual total pressure of 300 kg. and the resistance of the charge is measured. In this powder test the usual calcined but ungrapln'tized material has resistance of at least 300 ohms per mm. /m. and may run as high as 700 ohms per mmF/m. depending upon the degree of calcination.

For the purposes of illustration, in the accompanying drawing I show a graph illustrating the change in resistance for petrol coke relative to the degree of graphitization.

As may be noted from this drawing, the coke selected 2 had a resistance of about 300 ohms with zero graphitization. It may be noted that at 10% of graphite this resistance has dropped to approximately 230 ohms and it continues to drop as the graphite content increases until it has dropped to about 100 ohms at 60% graphite content.

In carrying out my invention I use a coke which has been calcined to the point where it contains at least 10% of graphite as measured under the foregoing test and preferably one which has been graphitiz'ed to contain between 20% and 60% of graphite and having a powder resistance under the foregoing powder test of between 100 and 200 ohms per mm. /m. In this connection it is to be noted that the ohmic resistance of the material tested in powder form will be considerably higher than the ohmic resistance of an electrode made from such powder where particles are bonded together by a tar or pitch binder and then baked.

I have found that in making the electrode, while it is advantageous to have all of the calcined carbonaceous material at least partly graphitized, highly beneficial results are obtained if only the coarser grain fractions are graphitized. By coarser grain fractions I mean that portion of the carbonaceous material which has a diameter of between 1 and 20 mm. It is more important that this fraction be graphitized than to have the fines graphitized, but even better results are obtained when both have had the continued calcination. The coarse grain fraction usually constitutes between 30% and 70% of the whole, the balance being fine material with a diameter of less than 1 mm.

In the following table I give diiferent characteristics of baked electrodes made in the first case from the ordinary, usual calcined anthracite. The second case is one in which the coarser fraction (from 1 to 20 mm.) is made from petrol coke which had been about 30% graphitized and the finer fraction used was the usual non-graphitized carbon. In the third case the sample was made from petrol coke in which both the grain fractrons were about 30% graphltlzed.

Table Modulus Bending El. Re- Thermic Dry Component of Elastic- Strength, sistance, Conductity, kg./cm. ohms ivity kg./crn. mrnfl/m.

(1) Oalcined anthra- 2.7 X 10 25-35 About Satisfaceite 3.5 X 10 tory. (2) Graphitized 2.3 X 10 51 64 Very good.

petrol coke in the coarse grain fractions (1-20 mm.) (3) Graphitized 2.6 X 10 45 48 Excellent.

petrol coke in all grain fractions From the foregoing table it is seen by comparison of 1 and 3 that 3 shows a reduction of about 33% in the electrical resistance and about 15% reduction of the modulus of elasticity, about 30% increase of bending strength and considerable increase in thermic conductivity. All three samples were baked according to the Soederberg system.

While I have referred to petrol coke, I can also use graphitized gas coke, metallurgical coke, or pitch coke instead of petrol coke. I have ascertained that the graph illustrated in the drawing applies to pitch coke as well as to petrol coke.

What is claimed is:

l. A process of producing electrode paste for use in continuous type electrodes for an electric furnace which consists in partially graphitizing coke until it shows a resistance on the powder test not greater than 230 ohms per mm. /m., equivalent to at least 10% graphitization, H

crushing such coke, separating out the portions of the coke having a diameter between 1 mm. and 20 mm., combining such material with fine carbonized carbon having a particle size of less than 1 mm. diameter and in an amount equal to between 30% and 70% of the total carbon, such fine carbon being selected from the group consisting of carbonized anthracite coal and carbonized coke, and a carbonaceous binder selected from the group consisting of tar and pitch.

2. A process as specified in claim 1, in which the fine material is also made from coke of which at least 10% is graphitized.

References Cited in the file of this patent UNITED STATES PATENTS Shea et a1. Oct. 31, 1950 Shea et a1. Aug. 7, 1951 

1. A PROCESS OF PRODUCING ELECTRODE PASTE FOR USE IN CONTINUOUS TYPE ELECTRODES FOR AN ELECTRIC FURNACE WHICH CONSISTS IN PARTIALLY GRAPHITIZING COKE UNTIL IT SHOWS A RESISTANCE ON THE POWDER TEST NOT GREATER THAN 230 OHMS PER MM.2/M., EQUIVALENT TO AT LEAST 10% GRAPHITIZATION, CRUSHING SUCH COKE, SEPARATING OUT THE PORTION OF THE COKE HAVING A DIAMETER BETWEEN 1 MM, AND 20 MM., COMBINING SUCH MATERIAL WITH FINE CARBONIZED CARBON HAVING A PARTICLE SIZE OF LESS THAN 1 MM. DIAMETER AND IN AN AMOUNT EQUAL TO BETWEN 30% AND 70% OF THE TOTAL CARBON, SUCH FINE CARBON BEING SELECTED FROM THE GROUP CONSISTING OF CARBONIZED ANTHRACITE COAL AND CARBONIZED COKE, AND A CARBONACEOUS BINDER SELECTED FROM THE GROUP 